We have developed a number of murine and rat monoclonal antibodies to guinea pig and mouse T lymphocyte cell surface antigens which play critical roles in the T cell activation process. One rat monoclonal antibody (7D4) which was prepared against a murine IL-2 dependent cell line inhibited the proliferation fo the immunogen to interleukin-2 (IL-2). Cell distribution studies demonstrated that the antigen defined by 7D4 was present on activated T and B lymphocytes, but was undetectable on normal murine lymphoid cells. Competition binding studies indicated that 7D4 failed to inhibit the binding of radiolabeled IL-2; however, 7D4 did precipitate radiolabeled IL-2 from detergent extracts of activated T cells that had been pulsed with IL-2. It is therefore likely that 7D4 detects an epitope on the IL-2 receptor distal to the ligand binding site or another molecule that physically associates with the receptor. A similar approach was used to prepare a murine monoclonal antibody (5C3) to a cloned line of guinea pig alloreactive T cells. 5C3 inhibited IL-2 driven proliferation as well as antigen and mitogen induced proliferation, but did not appear to define the guinea pig IL-2 receptor because it did not inhibit the binding of IL-2 and because T cell blasts that had been stripped of the antigen defined by 5C3 were not deficient in their expression of the IL-2 receptor. However, a role for the 5C3 bearing molecule in IL-2 driven growth was suggested by the observation that the 5C3-negative blasts were unable to proliferate to IL-2. The use of reagents such as 7D4 and 5C3 should greatly facilitate the analysis of the role of IL-2 as a universal growth hormone for T cells. These reagents are also attractive candidates for in vivo therapeutic use in attempts to selectively modulate or abrogate an ongoing immune response.